Lecture 6

Question 1

What are the names given to the two regions of the embryo lower order animals (not humans) that give rise to the three germ layers and extraembryonic structures

Answer 1

Epiblast – gives rise to the germ layers, hypoblast – forms vegetal pole/hemisphere

Question 2

In which regions of the morula would you find the structures that give rise to the embryo and to the extraembryonic structures

Answer 2

Top of the morula gives rise to the epiblast, bottoms gives rise to the hypoblast

Question 3

What changes govern the formation of these two layers in the morula

Answer 3

Governed by morphogens and transcription factors that lead to changes in gene transcription

Question 4

What causes the hollowing of the morula during development alongside squashing

Answer 4

Changes in osmolarity

Question 5

Which region of the morula ultimately ends up undergoing apoptosis

Answer 5

Hypoblast

Question 6

At which stage does the primitive streak in chick embryos form at the posterior of the embryo

Answer 6

Stage 3

Question 7

At stage 4 of chick development, the primitive streak has elongated and Hensen’s node has formed anteriorly, what does this structure then begin to express

Answer 7

Expresses and secretes BMP antagonists such as chordin, noggin and follastatin

Question 8

At what stage in chick development does the notochord begin to form

Answer 8

Stage 5-6

Question 9

What structure then extends forward from the node and is involved in anterior-posterior axis formation

Answer 9

The notochord

Question 10

Why are the events from morulation to gastrulation less studied in mammalian embryos and why is this

Answer 10

Mammalian embryos have been less extensively studied because they are difficult to maintain in culture after the blastula stage due to this being the stage at which they normally implant into the uterus

Question 11

At five days post-fertilisation what structure forms in the mammalian embryo that is similar to the hypoblast

Answer 11

Trophoblast

Question 12

What day corresponds to chick development stage 2 and corresponds to the embryo implantation into the uterus lining

Answer 12

12 days post-fertilisation

Question 13

At what stage in mammalian embryonic development is the embryo referred to as a blastula

Answer 13

12 day stage

Question 14

The processes occurring in chick embryos from stage 2-6 occur in 2-4 week old human embryos, T or F

Answer 14

T

Question 15

At which region of the developing embryo does gastrulation and germ layer formation begin at

Answer 15

Posterior of the embryo

Question 16

What is the first structure to become visible in the initial stages of gastrulation

Answer 16

Formation of the primitive streak due to cells moving into the midline forming a line that elongates anteriorly

Question 17

What forms at the most anterior point of the primitive streak

Answer 17

(Hensen’s – in chicks) node

Question 18

Which regions of the embryo does the primitive streak denote

Answer 18

Posterior/caudal regions

Question 19

The epiblast gives rise to mesoderm, endoderm and ectoderm, T or F

Answer 19

T

Question 20

Explain how signals from the hypoblast layer of the developing embryo accounts for the formation of three germ layers by the overlying epiblast

Answer 20

Signals from the hypoblast (trophoblast) induce some epiblast cells to become mesoderm and definitive endoderm. These cells involute and ingress along the anterior-posterior axis by losing contact with one another and migrating into the embryo. Superficial migrated cells become the mesoderm and the first cells to have ingressed, those now lying deeper within the epiblast will become endoderm.

Question 21

How does the migration of cells into the embryo at the primitive streak account for the formation of both mesoderm and endoderm

Answer 21

Cells that migrated first from the epiblast layer will lie deeper within the embryo and will become the endoderm. Those that migrated later will be lying for superficially and will become mesoderm.

Question 22

Once primitive streak reaches maximum extension and forms Hensen’s node, a similar event takes place whereby cells ingress but begin to move directly anteriorly/rostrally, T or F

Answer 22

T

Question 23

What is different about the gastrulation of Xenopus embryos compared with mammal/human early embryonic development

Answer 23

The Xenopus embryo doesn’t compact to form a blastodisc as mammalian embryos do. It remains morula-like with an animal cap and a vegetal hemisphere

Question 24

What structures in the Xenopus embryo correspond to the epiblast and hypoblast seen in mammalian embryos

Answer 24

Animal cap – epiblast, vegetal hemisphere – hypo/trophoblast

Question 25

Explain how oocytes are already polarised before fertilisation

Answer 25

Particular determinants inside the egg have become localised in the vegetal hemisphere of the cell. This is due to gravity in Xenopus and an interaction with the placenta in mammals

Question 26

What is the result of the inherent polarisation of the egg prior to fertilisation

Answer 26

Different cytoplasmic determinants have sunk to one part of the egg. This leads to the cells that come from this region being different and having specific factors localised/activated

Question 27

In an early cleavage event, the vegetal and animal hemispheres of the now fertilised egg are separated, what is the effect of this on cytoplasmic determinant localisation

Answer 27

Cytoplasmic factors restricted to the vegetal part of the oocyte are now restricted to vegetal hemisphere cells

Question 28

What is the effect of the factors that become localised to the vegetal hemisphere cells of the embryo as a result of early cleavages in the embryo that separate the animal and vegetal cells

Answer 28

These factors localised in the vegetal part of the embryo bind to promoters for particular transcription factors and upregulation. In-turn, these particular transcription factors become solely expressed in vegetal hemisphere cells

Question 29

What is the name of the transcription factor that is specifically expressed and localised to vegetal hemisphere cells

Answer 29

VgT

Question 30

Describe what happens as a result of the localisation of a specific transcription factor in the nuclei of cells in the vegetal hemisphere of the early embryo

Answer 30

Localised VgT in the nuclei of cells in the vegetal hemisphere leads to its binding to the promoter of a gene called Nodal and upregulate its expression. The Nodal morphogen then leaves the cells of the vegetal hemisphere and binds to receptors on cells in the animal hemisphere

Question 31

What is the name of the morphogen expressed in the vegetal hemisphere cells that is upregulated as a result of VgT binding to its promoter

Answer 31

Nodal

Question 32

What is the effect of the morphogen produced by cells of the vegetal hemisphere

Answer 32

Nodal binds to receptor on cells in the animal hemisphere and dictates the mesoderm and endoderm formation via cell proliferation, migration and differentiation.

Question 33

What happens to cells at the top of the animal hemisphere as a result of VgT activity on its promoter

Answer 33

The Nodal morphogen produced as a result of this leaves the cells and binds to receptors in the animal hemisphere. It forms a concentration gradient throughout the animal hemisphere. Cells at the top of the animal hemisphere don’t receive nodal signalling and so remain as ectoderm. This is thought to be due to either not expressing the nodal receptors or by not receiving a threshold concentration of the nodal ligand

Question 34

Nodal is expressed throughout the vegetal hemisphere/hypoblast in the developing embryo. What aspects/axes of early embryonic development does this signalling mechanism not account for

Answer 34

Can’t account for anterior-posterior axis formation and the appearance of the primitive streak at the distal region of the embryo

Question 35

Which signalling pathway is therefore responsible for anterior-posterior patterning and formation of the primitive streak

Answer 35

Wnt signalling